Hey so Ive got a 69 F100 with a 390 block that has been bored .60 over and the original 2 barrel carb with manual choke. Recently put hooker headers on it with about 5 feet of open straight pipe and it has been back firing a lot. Does it at cold and warm idle. Just wondering if that is normal for the set up. Also I've got it timed at about 10 degrees and carb screws adjusted about 2 turns out. wondering what the timing should be set at with that bore and what the carb should be adjusted to. I do have a timing light but no vacumm guage for adjusting the carb. Any guidance is apreciated, Thanks.

 

Might be running a tad too rich and blowing some fuel vapors out the exhaust. They would easily light and cause a backfire.

 

I think it needs mufflers. Right now you have ZERO exhaust back pressure. Gas engines like a littleback pressure unlike diesels. Read your spark plugs. It may be idleing lean. As far as the screws goes,no engine has a set amount of adjustment. At two turns I think it's lean. So adjust them equally counterclockwise untill you get the best and smoothest idle.

 

Looks like 351 was typing about the same time as me. I agree with him if there is a unburned rich mixture in the exhaust system. I was coming at it from a different direction since the idle mixture screws only out two turns. So read your plugs. It will tell ya.

 

Thanks guys, I will check the plugs here in a bit and report back. Yea ive been curious on the carb adjustment. When my dad owned the truck he said when it was all tuned up it got about 14mpg but right now im only getting about 7mpg. Curious how adjust that.

 

While you got the plugs out ya might do a compression test. Hopefully you don't have the burned exhaust valves I had in my head.

 

Good idea I will be doing that tonight after work. Thanks

 

After checking plugs for a lean condition (whitish)looking =lean

Check the carb main jet size and report back.
A stock 2barrle motorcraft carb for like a f250 c/s use between #-54-57 360/390
Now it's bored .060 over & headers I'd say you need to go up about 2-3 sizes.

Another way to check for a lean running motor is place your hand over the carb and slowly cut the air off with your hand and if the motor picks up speed it's running lean.
Pulling the choke out work the same way...
Orich

 

I pulled the plug for cylinder 1 and it looks a little whiteish so assuming its running a little lean. I adjusted the carb screws out 3/4 of a turn each from where i had it and it sounds much better. Purring instead of putting. I didn't have the time to check the jet as of right now but i will do that soon. And yes sorry i meant .060 bore. missed a zero. but since the carb was recently rebuilt with a kit it prolly has the smaller stock jet size. Thanks for your input and when I take a look at the jet I'll report back. Appreciate it!

 

I've got a number of jets ford extra jets so may be, I have some that will work for you.

Old skool rule was go up 3 sizes then adjust from there. As long as your not running lead.

I've had to drill them when waiting on jets ordered from ford back in the 70's..
I wouldn't run very high timing until you get the jets right.
Backing the air/fuel jets out leans it.

Orich

 

You did not say how the motor ran before installing the headers & long tubes?

You may be also have bad exh. seats like Jeff said to check.
Well be waiting for your test compression report:
Orich

 

No I'm not running lead. Pump gas 87 octane. And I've got the timing set at 6btdc which I believe is what the engine is supposed to be at. It ran alright before the headers. Although when I set the timing the very first time I found it was set to 16btdc which I thought was high. It didn't backfire like it does now but it had a harder time running it seemed. Started hard and felt sluggish. It had the stock ford exhaust manifolds on it which had cracked in pieces on both sides due to the harsh winters in fairbanks Alaska. Put on the headers and pipes and it seems to run better and have a little more power and umph. Starts way easier too. Just back fires now. I should be doing a compression test tomorrow after work so I will let ya know the results.

 

It looks like, I encountered a brain fart an said lead instead "lean" sorry for any misunderstanding from that.

A time soon turning 71 next month it seems to be happing more often!

Yea to far advanced can put a spark lock down on trying to start the motor.

1. The dizzy points are set first 0.017 with out a smog pump and with 0.022

2. Ign. timing should be set at when it's idling rpm at 550 auto tranny 600 for a standard trans. with Vac. advance hose removed from dizzy and plugged as not to suck any air.

3. Then carb adjustments the 2 air/fuel screw jets out for more air= Lean & in more gas..

Orich

 

Ah I see now, no worries. It happens to all of us.
Points are one thing I haven't checked but will check the gap on tonight. I think i have a spare set around if need be.
Ive got the carb screw adjusted out about 2 3/4 turn each and it seems like it's running good. I dont know what RPM im running since I dont have a tach or digital timing light. I have a standard tranny and it sounds like it's running in the 600 to 700 range right now.

Thats how ive been setting the timing so Ive got that down.

 

Sad day in the shop today.
Did the compression test, here are the results:

Cyl 1: 82 psi
Cyl 2: 130 psi
Cyl 3: 5 psi
Cyl 4: 120 psi
Cyl 5: 5 psi
Cyl 6: 10 psi
Cyl 7: 120 psi
Cyl 8: 130psi

So I'm running on about 4 1/2 cylinders. Looks like I'm gunna be pulling the heads and rebuilding or replacing if cracked here soon.